streamclone: move payload header generation into own function
The stream clone data over the wire protocol contains a header line
indicating total file count and data size. In bundle2, this metadata can
be captured by a part parameter and doesn't need to be in the body.
In preparation for bundle2, have generatev1() return the raw metadata
and move the header generation to its own function.
When Mercurial accepts more than one revision, they may be specified
individually, or provided as a topologically continuous range,
separated by the ":" character.
The syntax of range notation is [BEGIN]:[END], where BEGIN and END are
revision identifiers. Both BEGIN and END are optional. If BEGIN is not
specified, it defaults to revision number 0. If END is not specified,
it defaults to the tip. The range ":" thus means "all revisions".
If BEGIN is greater than END, revisions are treated in reverse order.
A range acts as a closed interval. This means that a range of 3:5
gives 3, 4 and 5. Similarly, a range of 9:6 gives 9, 8, 7, and 6.